Led array systems

ABSTRACT

A light emitting diode (LED) array comprises an array of LEDs mounted to a substrate. The LEDs emit light in a direction generally perpendicular to the substrate. An optical sheet is disposed over the LEDs. At least a portion of light entering one side of the optical sheet from the LEDs is guided within the optical sheet in a direction generally parallel to the substrate. Light extraction features direct light from the optical sheet in a generally forward direction. Such an array is useful for several applications, including space lighting, direct information display and backlighting of liquid crystal displays. The light spreading effect of the optical sheet reduces the amount of black space between LED pixels.

FIELD OF THE INVENTION

The invention relates to lighting or display assemblies, and moreparticularly relates to lighting or display assemblies that use arraysof light emitting diodes (LEDS).

BACKGROUND

LED arrays are typically formed using LEDs that have a polymerencapsulant formed over an LED die and an associated reflector cup. TheLED die itself has a quasi-Lambertian emission pattern and much of thelight generated within the LED die is trapped due to total internalreflection at the die surface or emitted from the edge. The polymerencapsulant is shaped to extract more light and to focus the extractedlight into a preferred emission profile. The reflector cup capturesedge-emitted light and bends it forwards. The array, commonly formed asa tile, may also have side reflectors. The total effect of thereflectors and the encapsulant controls the optical profile of the LEDpixel in the array.

Traditionally, the manufacture of a LED array thus includes steps forencapsulating the LED die within an encapsulated package, and thenlocating and placing the encapsulated package within the array. Theoptics of the array derive from the package and additional featuresformed on the array tile. A display, or illumination system, soconstructed, consists of point sources of light that knit togethervisually from a long observation distance. However, the point sources oflight break apart visually at shorter distances. Manufacturing isinherently inefficient due to the separate packaging and arraypopulation steps and the optics suffer due to the nature of the pointsource within an extended array. Furthermore, the resulting array tileis relatively thick and bulky.

SUMMARY OF THE INVENTION

The invention described herein is particularly useful for themanufacture and use of LED arrays that are used for lighting purposes orfor information display.

According to one embodiment of the invention, an optical assembly foremitting light comprises an array of light emitting diodes (LEDs)mounted to a substrate. The LEDs emit light in a direction generallyperpendicular to the substrate. An optical sheet is disposed over theLEDs. At least a portion of light entering one side of the optical sheetfrom the LEDs is guided within the optical sheet in a directiongenerally parallel to the substrate.

Another embodiment of the invention is directed to a light emittingsystem having a plurality of individually illuminated light emittingelements. The system comprises an array of light emitting diodes (LEDs),different LEDs corresponding to respective light emitting elements ofthe light emitting system. A light spreader sheet is disposed over theLEDs. Light entering the light spreader sheet from the LEDs is spreadtransversely within the spreader sheet over an area corresponding to therespective light emitting elements of the light emitting system. Thelight spreader sheet comprises light directing features that direct thespread light out of the spreader sheet.

Another embodiment of the invention is directed to a light emittingsystem having a plurality of individually illuminated light emittingelements. The system comprises an array of LEDs emitting light generallyin a light emission direction, light spreading means for laterallyspreading light in a direction across the array of the LEDs, and lightdirecting means for directing light from the light spreading means in adesired illumination direction.

Another embodiment of the invention is directed to an assembly foremitting light. The assembly comprises an array of LEDs arranged on asubstrate to emit light generally in a light emission direction. Anarray of reflectors is disposed with the LEDs. The reflectors defineindividual portions of a reflector sheet. The reflectors have respectiveapertures and respective LED of the array of LEDs protrude through therespective apertures. The substrate is positioned to a first side of thereflector sheet, and light emitting surfaces of the LEDs are beingpositioned to a second, reflecting side of the reflector sheet. A screenlayer is disposed on the second side of the reflector sheet, at leastsome of the light from the LEDs being directed by the screen layer afterreflecting off the reflectors.

The above summary of the present invention is not intended to describeeach illustrated embodiment or every implementation of the presentinvention. The figures and the detailed description which follow moreparticularly exemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 schematically illustrates a lighting unit according to anembodiment of the present invention;

FIG. 2 schematically illustrates an illuminated panel of the lightingunit of FIG. 1;

FIGS. 3A-3C show details of an illuminated panel according to anembodiment of the present invention in increasing detail;

FIG. 4A schematically illustrates an exploded view of an embodiment of alighting panel according to principles of the present invention;

FIGS. 4B-4F schematically illustrate exploded views of other embodimentsof lighting panels according to principles of the present invention;

FIGS. 5A-5D show schematic cross-sections through different embodimentsof light emitting elements of a lighting unit according to principles ofthe present invention;

FIG. 6 schematically illustrates a cross-section through a lightemitting element of a lighting unit, where an embodiment of a lightextraction element includes prismatic elements on a guiding layer,according to principles of the present invention;

FIG. 7 schematically illustrates a cross-section through a lightemitting element of a lighting unit, where another embodiment of a lightextraction element includes prismatic elements on a guiding layer,according to principles of the present invention;

FIG. 8 schematically illustrates a cross-section through a lightemitting element of a lighting unit, where another embodiment of a lightextraction element includes prismatic elements on a guiding layer,according to principles of the present invention;

FIG. 9 schematically illustrates a cross-section through a lightemitting element of a lighting unit, where another embodiment of a lightextraction element includes indentations on a guiding layer, accordingto principles of the present invention;

FIG. 10A schematically illustrates a cross-section through a lightemitting element of a lighting unit, where another embodiment of a lightextraction element includes prismatic elements on a guiding layer,according to principles of the present invention;

FIG. 10B schematically illustrates an embodiment of a light emittingelement like that illustrated in FIG. 10A along with a screen layer,according to principles of the present invention;

FIGS. 11 and 12 schematically illustrate embodiments of a light emittingelement that incorporates light management films, according toprinciples of the present invention;

FIG. 13 schematically illustrates an embodiment of a light emittingelement having a guiding layer with an upper surface that isnon-parallel to the substrate, according to principles of the presentinvention;

FIG. 14A schematically illustrates an embodiment of a light emittingelement having a guiding layer with a lower surface that is non-parallelto the substrate, according to principles of the present invention;

FIGS. 14B and 14C schematically illustrate structures that may be usedon the lower surface of the guiding layer of the light emitting elementin FIG. 14A, according to principles of the present invention;

FIGS. 15A and 15B schematically illustrate cross-sections through alight emitting element of a lighting unit, where other embodiments of alight extraction element include structure on a layer below the lightguiding layer, according to principles of the present invention;

FIGS. 16A-D schematically illustrate embodiments of patterns that may beused for the light extraction elements in a light emitting element,according to principles of the present invention;

FIGS. 17A-D schematically illustrate embodiments of light emittingelements that have optical barriers between adjacent light emittingelements, according to principles of the present invention;

FIGS. 18A and 18B schematically illustrate cross-sections throughreflective light emitting elements according to principles of thepresent invention; and

FIGS. 19A and 19B schematically illustrate cross-sections throughembodiments of light emitting elements having diffuser associated withthe guiding layer, according to principles of the present invention; and

FIGS. 20A-20D schematically illustrate cross-sections throughembodiments of light emitting elements having a layer of diffuselyreflecting material on the viewing side of the guiding layer, accordingto principles of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is applicable to lighting units, and is moreparticularly applicable to lighting units that provide illuminationusing light emitting diodes (LEDs). The lighting units may provide lightfor illuminating an area or may provide information to a viewer byselective illumination of different areas of the lighting unit, as in aninformation display.

An example of an addressable lighting unit 100 that uses LEDs isschematically illustrated in FIG. 1. A power supply 102 supplieselectrical power to the display panel 104. The display panel 104comprises an array of individually addressable light emitting elements.The power supply 102, or control circuits within the panel 104,selectively provides current to individual light emitting elements so asto illuminate a desired pattern on the display panel 104.

Part of the display panel 104 is shown in FIG. 2. As the viewer movescloser to the display panel 104, the individuality of the light emittingelements 106 becomes more apparent. If the viewer moves sufficientlyclose to the panel 104, it becomes easier for the viewer to perceive theindividual light emitting elements of the display panel, and moredifficult to perceive the message being displayed on the panel. Forexample, the individual elements 106 form a “T” shape, but the “T” isnot as easily discerned as the view of the “T” from further away.

Part of the reason for this problem is that the individual lightemitting elements 106 on the display are separated from each other by asignificant black border and so, at close distances, it becomes easierfor the viewer to see the light emitting elements as separate elements,rather than part of a pattern. For comparison, the display of the letter“T” in the panel 200 demonstrates the enhancement in the readability ofinformation where the extent of the dark space separating adjacent lightemitting elements 206 is reduced.

In another embodiment, the lighting unit 100 may simply illuminate allthe lighting emitting elements so as to provide as much light aspossible. Such a lighting unit may be used for lighting purposes, ratherthan for information display.

An embodiment of the structure of the lighting unit 300 is furtherexplained with reference to FIGS. 3A-3B. The display panel 302 of thelighting unit 300 comprises a number of tiles 304, and each tile 304comprises a number of light emitting elements 306. The layers of thetile 306 comprise a substrate 308, a guiding layer 310 and an optionalcontrast/projection filter 312.

The contrast/projection filter 312 is used to i) reduce the amount ofreflected ambient light and/or ii) provide viewing angle so as tooperate as a projection screen. The selection of ambient light reductionand viewing angle depends on the particular application of the lightingunit, whether there is significant ambient light that needs to berejected or whether it is desirable to spread the light from thelighting unit over a wide angle. For example, where the lighting unit isused in outdoor information display applications, it may be desirable toinclude both ambient light reduction, in order to improve the contrastunder direct sunlight, while also providing a wide viewing angle so asto increase the area in which a viewer can see the information. Inanother example, where the lighting unit is used indoors, there may beno need to include ambient light rejection.

Where the contrast/projection filter 312 reduces the reflection ofambient light, the contrast/projection filter may includeanti-reflection properties, for example an anti-reflection layer, toreduce the amount of reflection of the ambient light. The filter 312 mayalso, or alternatively, include anti-glare properties that reduce thespecular reflection of the ambient light, for example a matte surface.Another approach to reducing the amount of reflected ambient light is toabsorb the ambient light. Where the filter 312 provides viewing angle,the filter may include one or more layers that operate as a projectionscreen, for example a lenticular projection screen, or a beaded screen.Some lenticular and beaded screens provide both ambient light reductionand viewing angle. One example of such a screen, illustrated in FIGS. 3Band 3C, is a Vikuiti™ XRVS type screen available from 3M Company, St.Paul, Minn. The XRVS screen provides viewing angle by refracting thelight through transparent spheres embedded in a layer of absorbingmaterial: the transparent spheres provide a low loss path for the lightto pass through the screen while the absorbing material absorbs theincident ambient light. Other types of filters may be included in thecontrast/projection filter, for example a polarizer layer, such as areflective polarizer or an absorbing polarizer, that transmits light ina desired polarization state. Other types of filter that may beincorporated include a Vikuiti™ Circular Polarizer (CP) layer, or aVikuiti™ Light Control Film layer that provides privacy from off-axisviewing and/or provides reduction in the amount of ambient light that isreflected from the display. Both Vikuiti™ layers are available from 3MCompany.

At the level of an individual light emitting element 306, one or moreLEDs 320 are disposed on the substrate 308. If only a single color needsto be emitted from the light emitting element 306, then only a singleLED may be used, or more than one LED of the same type may be used toaugment the optical power emitted by the light emitting element 306.Different LEDs may be used to produce the different colors where thecolor of light emitted from the light emitting element is selectable.Individual control of the different LEDs leads to the ability to controlthe color of the emitted light. In addition, if it is desired that thelight emitting element 306 emit white light, then the light emittingelement 306 may be provided with a number of LEDs emitting light ofdifferent colors, whose combined effect is to emit light perceived by aviewer to be white. Another approach to producing white light is to useone or more LEDs 320 that emit light at a relatively short wavelengthand to convert the emitted light to white light using a phosphorwavelength converter. White light is light that stimulates the red,green, and blue sensors in the human eye to yield an appearance that anordinary observer would consider “white”. Such white light may be biasedto the red (commonly referred to as warm white light) or to the blue(commonly referred to as cool white light). Such light can have a colorrendering index of up to 100.

The term LED is used to refer to different forms of inorganicsemiconductor light emitting diode formed, for example, from acombination of one or more Group III elements and of one or more Group Velements (III-V semiconductor). Examples of III-V semiconductormaterials that might be used in an LED include nitrides, such as galliumnitride or indium gallium nitride, and phosphides, such as indiumgallium phosphide. Other types of III-V materials may also be used, asmight inorganic materials from other groups of the periodic table.

The LEDs may be packaged LEDs or non-packaged LEDs, for example, LEDdies, surface-mounted LEDs, chip-on-board LEDs and LEDs of otherconfigurations. The term LED also includes LEDs packaged or associatedwith a phosphor where the phosphor is used to convert light emitted fromthe LED to light at a different wavelength. Chip-on-board (COB) is ahybrid technology that employs face-up-bonded chip devicesinterconnected to a substrate conventionally, for example using wirebonding. Connections may be made by wire bonding, tape automated bonding(TAB), or flip-chip bonding. The examples illustrated herein mostly showLED dies, but this is not intended as a limitation, and other types ofpackaged LED, as described in this paragraph, may also be used.

A reflective layer, 322 may be provided on the substrate 308 to directlight emitted from the LED 320 towards the viewer. Also, the guidinglayer 310 may include a number of light extraction features 324 forextracting light from the film and directing the light towards theviewer, through the optional screen layer 312. The light extractionfeatures 324 may be arranged in a radial pattern on the guiding layer310, centered on or about the LED 320. The optical path followed bylight emitted from the LED 320 may include reflection within the guidinglayer 310 before being directed to the viewer. The tile structure may beeliminated if the manufacturing processes permit the manufacture ofsufficiently large substrate, lens films and screen layers.

A partial exploded view of a lighting unit 400 is schematicallypresented in FIG. 4A. The lighting unit 400 comprises a substrate 408 onwhich are arrayed a number of LEDs 420. The substrate 408 may be formedfrom any suitable type of material. For example, the substrate 408 maybe formed from a metal, a ceramic or a polymer. One particular exampleof a polymer substrate is polyimide, such as Kapton-brand polyimidemanufactured by Du Pont, Wilmington, Del. The substrate 408 may beflexible or may be rigid. The substrate 408 may also be formed from atransparent material, such as polycarbonate, for example as manufacturedby GE Plastics, Pittsfield, Mass.

An intermediate layer 422 may be introduced between the substrate 408and the guiding layer 410. The guiding layer 410 is typicallytransparent to the light emitted by the LED 420 and may be formed, forexample, from a transparent polymer, such as a polycarbonate, apolyester, a urethane, an acrylate or the like. This list of polymermaterials is not intended to be an exhaustive list of suitable polymermaterials.

The guiding layer 410 may include an array of light extraction elements424 associated with each LED 420. The intermediate layer 422 includesapertures 423, also referred to as vias, that are registered to the LEDs420 disposed on the substrate 408. The intermediate layer 422 may bereflective for light at the wavelength emitted by the LED 420. Theintermediate layer 422 may comprise, for example, a multi-layer polymerreflective film such as Vikuiti™ ESR film available from 3M Company, St.Paul, Minn. The intermediate layer 422 may also be a white diffusereflector such as a matrix containing diffusely reflecting particles,for example titanium dioxide particles. It will be appreciated that theintermediate layer 422 may also include some other type of reflector,such as a metalized layer or multilayer dielectric coating. Theintermediate layer 422 may be bonded to the substrate 408, for exampleusing a pressure sensitive adhesive. Apertures may be formed in theintermediate layer 422 using, for example, laser milling.

In another embodiment, the intermediate layer 422 may be non-reflecting.This is particularly useful where the lighting unit is used inapplications with high amounts of ambient light. The non-reflectingintermediate layer 422 may be formed from a polymer layer that includesan absorbing species, such as carbon particles, distributed within thepolymer matrix. The non-reflecting intermediate layer helps to increasethe viewability of the light emitted from the LEDs 420 by reducing theamount of ambient light reflected by the lighting unit.

Several different approaches are available for forming the guiding layer410 over the LEDs 420. One example is to laminate the guiding layer 410over the LEDs 420 and/or the intermediate layer 422 as a sheet withpre-prepared light extraction elements. In another example, the guidinglayer 410 may be formed by coating a polymer layer over the LEDs 420and/or the intermediate layer 422 and by forming the light extractionelements in situ. Presentation of these two examples is not intended topresent an exhaustive list of approaches to forming the guiding layer410 over the LEDs 420.

Conductors may be provided on different layers for carrying electricalcurrent to and from the LEDs 420. For example, conductors may beprovided on any of the substrate 408, the intermediate layer 422 and/orthe guiding layer 410 to carry current to and from the LEDs 420. Theconductors may take the form of metallic traces, for example formed fromcopper. In the example illustrated in FIG. 4A, conductors 421, arepositioned on the substrate 408 to carry current to and/or from the LEDs420. The LED 420 is wire bonded to a conductor 425 on the substrate 408via a wire bond 426. The LED 420 may also be of the flip-chip variety,with both electrodes formed on its lower surface. Electrical connectionsmay be made to electrical conductors using any suitable technique, suchas solder reflow, or connection using a conductive epoxy such as Metechtype 6144, available from Lord Corp., Cary, N.C.

The LED 420 may be electrically connected to conductors provided onother layers, as is now discussed with regard to FIGS. 4B-4E. FIG. 4Bschematically illustrates an example where conductors 421 are providedon the substrate 408 and other conductors 431 are positioned on thelower surface of the intermediate layer 422. FIG. 4C schematicallyillustrates an example where conductors 421 are provided on thesubstrate 408 and other conductors 441 are positioned on the uppersurface of the intermediate layer 422. FIG. 4D schematically illustratesan example, where conductors 421 are provided on the substrate 408 andother conductors 451 are positioned on the lower surface of the guidinglayer 410.

In another approach, schematically illustrated in FIG. 4E, the layer 460may be provided with both conductors, 461 and 462, which may be providedon different sides of the layer 460 or on the same side of the layer460. The layer 460 contains an aperture or recess 423 that accepts theLED (not shown). The layer 460 may be a substrate layer, for example,where the substrate is formed of a reflective material, or may be anintermediate layer, in which case a substrate layer (not shown) may beprovided below the layer 460. The conductor 461 may cover the opening ofthe aperture 423, so as to make the aperture 423 a blind hole, whichholds the LED in place in the aperture 423.

The substrate 408 may be provided with a metallic layer on its lowersurface (not shown) for extracting heat generated by the LEDs 420. Inaddition, the conductors 421 may be provided with large area pads 421 ato aid in spreading the heat generated by the LEDs 420. Generally, wherethe conductors 420 do not lie in the optical path between the LEDs 420and the viewing space, the dimensions of the conductors 420 may belarger so as to aid in spreading the heat from the LEDs 420. Where theconductors are positioned in the optical path, however, for example onthe lower surface of the guiding layer 410 or on the upper surface of areflective intermediate layer 422, it is generally desirable to reducethe size of the conductors so as to reduce adverse effects on lightpassing to the viewing space.

The LEDs 420 may be arranged on the substrate 408 in a rectangularpattern, or square pattern, as illustrated. This leads to easy displayof vertical and horizontal lines in an information display application.A rectangular or square pattern is not required, however, and the LEDs420 may be laid out on the substrate 408 in some other pattern, forexample in a hexagonal pattern. The actual shape of the light emittingelement may be square, rectangular, round or some other shape. Theinterstices between light emitting elements, in other words those areasbetween light emitting elements where there is little or no lightemitted, may be used as bonding surfaces, for example to fastenbrightness enhancing films or projection filters.

Although only one LED 420 is associated with each light emitting elementin FIGS. 4A-4D, there may be more than one LED 420 associated with eachlight emitting element. For example, the LED 420 may comprise aplurality of LEDs, such as LED dies 470 mounted close together andemitting different colors, as schematically illustrated in FIG. 4F. Suchan arrangement permits not only the production of a mixed light color,such as white light, it also permits the user to control the shade ofthe color emitted by controlling the relative amounts of light emittedfrom each LED.

In the examples described below, the LEDs are illustrated in the form ofchips (dies) that are directly mounted to the substrate. This is notintended as a limitation of the invention and other forms of LED mayalso be used.

A cross-section through one particular embodiment of a light emittingelement 500 is schematically illustrated in FIG. 5A. In this embodiment,the LED 520 is bonded to the substrate 508. The intermediate layer 522has a height approximately the same as the LED 520 and the guiding layer510 may be bonded to both the LED 520 and the intermediate layer 522.Where there is a wire bond between the LED 520 and the substrate 508,the wire bond 529 may pass between the LED 520 and the substrate in aseparate via 528 through the intermediate layer 522. The wire bond 529may also pass within the same via 523 as the LED 520.

While the light from the LED is emitted over a wide range of angles, theLED 520 is positioned to direct light generally upwards, in a directionaway from the substrate 508. An LED axis 520 a is shown lyingperpendicular to the substrate 508: many LEDs emit light symmetricallyabout the LED axis 520 a. Where the light is not emitted symmetricallyabout the LED axis, 520 a, the LED axis corresponds to the averagedirection along which light is emitted from the LED 520. The axis 520 aneed not represent the direction of maximum light intensity emitted bythe LED 520.

A cross-section through another embodiment is schematically illustratedin FIG. 5B. In this embodiment, the LED 520 is higher than theintermediate layer 522 and stand-offs 530 may be used to hold theguiding layer 510 above the intermediate layer 522. The stand-offs 530may hold the guiding layer 510 at a height that contacts the LED 520 ormay hold the guiding layer 510 at a height out of contact with the LED520.

A cross-section through another embodiment is schematically illustratedin FIG. 5C. In this embodiment, the height of the intermediate layer 522is less than the height of the LED 520. Stand-offs 532 are providedbetween the intermediate layer 522 and the substrate 508. Theintermediate layer 522 may be bonded to the lower surface 511 of theguiding layer 510.

The LED need not be square or rectangular in cross-section. For example,the LED 520 may have a different shape, as is schematically illustratedin FIG. 5D. Furthermore, the aperture in the intermediate layer 522 maybe sized larger than the LED 520 so as to reduce the constraints onregistration between the LEDs 520 and the intermediate layer 522 whenassembling the lighting unit.

A cross-section through another embodiment is schematically illustratedin FIG. 5E, in which a portion of the LED 520 sits within a recess 550in the guiding layer 510. In this embodiment some of the light 552 thatis emitted from the side of the LED 520 is directed into the guidinglayer 510 and is guided along the guiding layer 510. Light 554 that isemitted from the top of the LED 520 may also be guided along the guidinglayer 510.

Optical coupling between the LED 520 and the guiding layer 510 may befacilitated through the use of an optical coupling material 540 disposedbetween the LED 520 and the guiding layer 510. The optical couplingmaterial 540 may also provide adhesive properties that increasestructural integrity.

The optical coupling material 540 may be applied in different waysduring the assembly process. In one approach, the coupling material 540is disposed on top of the LEDs 520 prior to application of the guidinglayer 510. The guiding layer 510 is then applied over the assembly ofLEDs 520. Such application typically displaces the coupling material 520so as to spread into the vias 523 containing the LEDs 520 and alsobetween the intermediate layer 522 and the guiding layer 510. In anotherapproach, the guiding layer 510 is applied over the LEDs 520, and thenthe coupling material 540 is permitted to wick in between the guidinglayer 510 and the intermediate layer 522 and LEDs 520 through capillaryaction. In this approach, the coupling material may also fill, orpartially fill, the vias 523 containing the LEDs 520. One example of theoptical coupling material 540 is Norland type NOA 81 optical adhesive,supplied by Norland Products, Cranbury, N.J.

Different approaches to distributing the light from the LED are nowdescribed with reference to FIGS. 6-18. In many of these approaches,some of the light from the LED is reflected within the guiding layer510, at either the lower surface or the upper surface, or both, and sothe guiding layer 510 may be said to guide the light from the LED.Laterally guiding the light away from the LED, also referred to asspreading the light, before directing the light to the viewer may helpto reduce the amount of dark space between adjacent light emittingelements of the lighting unit. It will be appreciated that not all thelight emitted by the LED 520 need be guided within the guiding layer510.

In FIG. 6, an LED 620 is disposed on a substrate 608 and is surroundedby an intermediate layer 622, which may be a reflecting film. A guidinglayer 610 lies over the LED 620. Light from the LED 620 passes into theguiding layer 610. The side of the guiding layer 610 facing away fromthe LED is provided with a light extraction feature 624. In thisparticular embodiment, the light extraction feature 624 includesprismatic structures 626 disposed about the LED 620. The prismaticstructures 626 may be formed as a Fresnel lens with the LED 620 at itscenter. Light 630 from the LED 620 passes into the guiding layer 610.Some of the light 632 may be emitted directly via the prismaticstructure 626. Other portions of the light 634 may be reflected withinthe guiding layer 610, for example by reflection off the upper surface611 of the guiding layer 610. The light 634 may also reflect off thelower surface 612 of the guiding layer 610 or off the intermediate layer622. The light 634 is directed in a forwards direction towards theviewer by the prismatic structures 626.

In this and the following embodiments, the light may be reflected backinto the guiding layer 610 by the intermediate layer 622, or may beinternally reflected and/or refracted at the lower surface 612 of theguiding layer 610. Where the light is internally reflected at the lowersurface 612, light extraction features may also be provided on the lowersurface 612. Where the light is reflected by the intermediate layer 622,then light extraction features may also be provided on the intermediatelayer 622 and/or on the lower surface 612.

In the embodiment schematically illustrated in FIG. 7, the lightextraction features 624 may comprise areas that include diffractivestructures or diffusely reflecting material 636 provided on the uppersurface 611 of the guiding layer 610. Light 630 is guided within theguiding layer 610. However, after diffraction or diffuse reflection offthe diffractive structures or diffusely reflecting material 636, thelight 630 is directed to the intermediate layer 622, which reflects thelight back through the guiding layer 610 towards the viewer. The densityof the diffractive structures or diffusely reflecting material 636 onthe surface 611 of the guiding layer 610 may be selected so as to directlight to the intermediate layer 622 while also permitting light to passthrough the upper surface 612 towards the viewer.

In the embodiment schematically illustrated in FIG. 8, the lightextraction features 624 comprise extraction grooves 646 that penetratethe upper surface 611 of the guiding layer 610. The extraction grooves646 help to direct light from the upper surface 611 to the intermediatelayer 622 which reflects the light back through the guiding layer 610 tothe viewer.

In the embodiment schematically illustrated in FIG. 9, the lightextraction features comprise indentations 648 that penetrate the uppersurface 611 of the guiding layer. The indentations 648 may direct lightfrom the upper surface to the intermediate layer 622 at an angle thatpermits the light reflected from the intermediate layer 622 to betransmitted through the upper surface 611 of the guiding layer. Theindentations 648 may also permit light incident thereon to directly passout of the guiding layer 610. The indentations 648 and grooves 646,shown in FIG. 8, may be implemented on the guiding layer 610 in anypattern for extracting the light from the guiding layer to produce adesired illumination profile.

In the embodiment schematically shown in FIG. 10A, the light extractionfeature 624 comprises a layer of optical coupling material 650 and aplurality of prism-type structures 652 whose apexes penetrate into theoptical coupling material 650. The prism-type structure 652 may beattached to a base layer 654. For example, the prism-type structures 652may be molded with the base layer 654 or may be bonded to the base layer654. The optical coupling material 650 may be, for example, a thin layerof adhesive with a thickness in the range 1-20 μm. In this particularembodiment, the light 630 may pass out of the guiding layer 610 to betotally internally reflected at the upper surface 651 of the couplingmaterial 650. The light 630 is directed to the intermediate layer 622which reflects the light back upwards. Some of the light incident on theupper surface 612 of the guiding layer 612 is coupled into theprism-type structures 652, which reflect the light upwards in thegeneral direction of the viewer. The base layer 654 may be provided witha surface or volume diffuser to diffuse the light 656 emitted from thebase layer 654. The prism-type structures 652 may be shaped in a patterncentered around the LED 620, or may be provided in some other pattern.For example, the prism-type structures may be linear.

In a variation of the embodiment illustrated in FIG. 10A, acontrast/projection filter 658 may be provided close to the output sideof the base layer, for example as is illustrated in FIG. 10B.

One or more light management films may be used above the guiding layerfor directing or redirecting the light. For example, one or morebrightness enhancing films, available from 3M Company, St. Paul, Minn.,under the trade name BEF™, may be used to direct the light more in adirection perpendicular to the substrate 608. Brightness enhancing filmstypically comprise a plurality of prismatic refractive elements which,when illuminated from the base side of the prismatic elements, refractthe transmitted light in a direction more parallel to the axis. In theexample illustrated in FIG. 11, a light emitting element that uses adiffractive light extraction element 636 emits light from the guidinglayer 610 in a first direction relative to the axis 660. The light 667is refracted upon exiting the first brightness-enhancing layer 662 in asecond direction that is closer to the axis 666 than the firstdirection. In some embodiments, only one layer of brightness enhancingfilm 662 is used. Typically, the prisms of the first brightnessenhancing film are ribbed, and so a single layer of brightness enhancingfilm redirects the light in only one dimension. A second additionallayer of brightness enhancing film 664 may optionally be used, where theribbed prisms of the second layer of brightness enhancing film 664 areoriented perpendicular to those of the first layer 662: the combinationof the two layers 662 and 664 of brightness enhancing film redirects thelight towards the axis 666 in two dimensions.

A contrast/projection filter 668 may be provided at the output from thelight management films 662 and 664, for example as is schematicallyillustrated in FIG. 12. For certain applications, a reflectivepolarizing layer may be provided above the light emitting element, forexample a multilayer optical film (MOF) reflective polarizer such asVikuiti™ DBEF film supplied by 3M Company, St. Paul, Minn. A reflectivepolarizer transmits light in one polarization state and reflects thelight in the orthogonal polarization state: such a polarizer may be usedwhen the light emitting element is used to provide polarized light, e.g.for backlighting an LCD display. The light in the polarization statethat is reflected by the reflective polarizer is reflected back towardsthe substrate 608. One of the layers, for example the intermediate layer622 may be treated to reflect the light reflected by the reflectivepolarizer in a manner that causes some depolarization, so that the lightreflected by the reflective polarizer may be recycled and eventuallypassed through the reflective polarizer.

The guiding layer 610 need not have parallel surfaces. In other words,the surfaces 611 and 612 may be nonparallel to each other. One suchembodiment is schematically illustrated in FIG. 13, in which the lightextraction feature is an upper surface 611 that lies non-parallel to thelower surface 612 and to the substrate 608. The light 630 internallyreflects at the upper surface 611 and is directed to the lower surface612, and is reflected by the lower surface or the intermediate layer 622as light 670. However, since the upper surface 611 is not parallel withthe lower surface 612, the reflected light 670 is not incident on theupper surface 611 at the same angle of incidence as the light 630 fromthe LED. Where the reflected light 670 is incident on the upper surface611 at an angle of incidence less than the critical angle, the reflectedlight 670 is transmitted through the upper surface 611 as light 672.Another layer (not shown), such as BEF™, or the like, may be used todirect the light 672 in a direction more perpendicular to the substrate608.

The guiding layer 610 may be provided with a component 674 for directingthe light from the LED 620 along the film 610, thus increasing theamount of the light that is guided by the guiding layer 610. In theillustrated embodiment, the component 674 includes a recess positionedabove the LED 620. Light 676 from the LED 620 is internally reflected atthe recess surface and is directed along the film 610. Where the angleof incidence on the recess surface is sufficiently high, the light istotally internally reflected. The light 676 may be directed out of thefilm 610 after reflection off the oblique upper surface 611. Thecomponent 674 may also be used in other embodiments of a light emittingelement discussed herein, where the guiding layer 610 has surfacesparallel to the substrate 608.

Another embodiment in which the surfaces of the guiding layer are notparallel is schematically illustrated in FIG. 14A. In this embodiment,the upper surface 611 is parallel to the substrate 608, but the lowersurface 612 is not parallel to the substrate 608. The light extractionfeature includes, in this case, the lower surface 612. The light 630 isinternally reflected by the upper surface 611 of the guiding layer 610and directed to the lower surface 612. The light is directed backtowards the upper surface 611 from the lower surface 612. When the lightfrom the lower surface 612 is incident on the upper surface 611 at anangle of incidence that is less than the critical angle, the light 680is transmitted through the upper surface 611.

Different types of reflector may be used on the lower surface 612. Forexample, the lower surface 612 may be coated with a reflecting film.Other approaches include providing structure on the lower surface 612.For example, the lower surface 612 may be provided with one or moresteps 682, as is schematically illustrated in FIG. 14B. The steps 682are set at a desired angle so as to totally internally reflect some ofthe incident light 686. In another approach, the lower surface 612 maybe provided with a series of elements 684, for example, prismaticelements that redirect the incident light 686 through a combination ofrefraction and total internal reflection. Light that is transmittedthrough the lower surface 612 of the guiding layer 610 may be reflectedback through the guiding layer 610 by the intermediate layer 622.

The extraction feature does not have to be situated on the upper surfaceof the guiding layer 610, but may be provided on the lower surface ofthe guiding layer 610 and/or on the intermediate layer 622. One exampleof such an arrangement is schematically illustrated in FIG. 15A, inwhich a reflective intermediate layer 622 is provided with a non-planarsurface structure 690. Some of the light 692 is guided by the guidinglayer 610 to be incident on the surface structure 690. Some of the lightreflected from the surface structure 690 is directed to the uppersurface 611 of the guiding layer 610, and out towards the viewer. Thesurface structure 694 may be introduced to the intermediate layer 622using a number of different techniques. Some approaches to forming astructure on a polymer reflector as may be used in the intermediatelayer 622 are described in U.S. Pat. Nos. 6,045,894 and 6,096,247, therelevant portions of which are incorporated herein by reference.

Another example is schematically illustrated in FIG. 15B, in which theupper surface of the reflective intermediate layer 622, and/or the lowersurface of the guiding layer 610, is provided with patches 695 ofdiffusely reflective light extraction material, such as a whitepigmented material. At least some of the light incident on the patches695 is not specularly reflected, but is diffusely reflected, whichallows some of the light 697 to be directed out of the guiding layer610. Other types of light extraction features may be provided on theupper surface of the intermediate layer 622 or on the lower surface ofthe guiding layer 610, and may also be provide in combination with lightextraction features on the top surface of the guiding layer 610.

The light extraction features described herein have been illustrated inexamples that contain only one type of extraction feature. It will beappreciated that the different types of light extraction featuresillustrated herein may be used alone, or in combination with other typesof light extraction features.

A significant fraction of the light produced by the LED 620 propagatesin a direction that might pass directly through the guiding layer 610,which may lead to a viewer perceiving a central bright spot in the lightemitting element. Other approaches may be used to spreading the lighttransversely, in addition to using a guiding layer. One such approachhas been discussed above with regard to component 674, illustrated inFIG. 13, in which the light is internally reflected at recess surfacesand thus directed generally along the plane of the guiding layer 610.After lateral spreading, the light is then extracted from the guidinglayer.

Another approach to spreading the light from the LED 620 within theguiding layer 610 is now discussed with reference to FIG. 19A. Adiffuser 1902, such as a volume diffuser, is positioned on the far sideof the guiding layer 610 from the LED 620. Where the diffuser 1902 is avolume diffuser, the volume diffuser may be located within the guidinglayer 610. The diffuser 1902 is used to reduce the amount of light thatpasses directly through the guiding layer 610 from the LED, and thus touniformize the output from the light emitting element. The diffuser 1902spreads some of the light 1904 laterally so that it can be extractedfrom the guiding layer at some other position via an extraction feature624.

The diffusion power of the diffuser 1902 may be spatially tailored so asto enhance the desired profile of light emitted from the light emittingelement. For example, where it is desired to reduce the amount of lightemitted above the LED 620, and to spread the light around the lightemitting element, the diffusing power of the diffuser 1902 is greatestabove the LED 620. In the illustrated embodiment, the diffuser 1902 isthickest above the LED 620 and the diffuser thickness 1902 tails offtowards the edge of the light emitting element 1900. The diffusing powerof the diffuser 1902 may also be adjusted by spatially varying thedensity of diffusing particles in the volume diffuser. In theillustrated embodiment, the thickness of the diffuser 1902 varieslinearly from the center of the light emitting element 1900.

In another embodiment, schematically illustrated in FIG. 19B, thethickness of the diffuser 1912 varies nonlinearly with position acrossthe light emitting element 1910. Furthermore, the diffuser 1912 may ormay not extend over the entire area of the light emitting element 1910.In the illustrated embodiment, the diffuser 1912 is does not extend overthe entire light emitting element 1910.

The light flux close to the LED tends to be high, with the result thatthe level of light emitted close to the LED can be significantly higherthan from points further away from the LED. An example, of such anemitted intensity profile is schematically shown as curve (a) in FIG.16A.

In order to obtain a more uniform intensity profile in the light emittedfrom the light emitting element, the light extraction features may beplaced or adapted to extract less light at a position close to the LEDitself, and to increase the amount of light extraction from thepositions of the light extracting element further away from the LED.This is illustrated further with reference to FIG. 16A, whichschematically illustrates a light emitting element 1600, showing theposition of the LED 1620 at the center of the light emitting element1600 and the light extraction features 1624 arranged radially around theLED 1620. In the illustrated embodiment, the density of the lightextraction features increases with radial separation from the LED 1620.Appropriate selection of the light extraction density may lead to a moreuniform emission intensity profile over the light emitting area, forexample as illustrated as curve (b) (dashed lines). In addition, theextraction strength of the extraction features may be varied withincreased distance from the LED 1620, so as to reduce the non-uniformityin light extraction over the light emitting element 1600. The extractionstrength may be adjusted by varying, for example, the size of theextraction feature.

All types of extraction features may be arranged in a manner thatcontrols the intensity profile of light emitted from the light emittingelement, including upper surface and lower surface extraction features,such as prismatic structures formed on the upper surface, diffractingstructures, extraction grooves, prismatic structures penetrating intothe upper surface and non-parallel surfaces on the guiding film. Inaddition, a combination of different types of extraction features may beused, such as a surface structure on the guiding film along withnon-parallel surfaces. The extraction features may be arranged toprovide a relatively uniform profile illumination profile or some otherdesirable profile. The term “uniform” refers to a relatively flatillumination profile where the area above the LED is not significantlybrighter than the surrounding area.

The light extraction features need not be arranged with radial symmetryaround the LED, but may be arranged in some other shape. One example ofsuch a shape is schematically illustrated in FIG. 16B, which shows thelight extraction features 1624 arranged in an approximately squarepattern around the LED 1620.

Light extraction features need not be arranged continuously around theLED, but may be discontinuous. One example of discontinuous lightextraction features 1634 is schematically illustrated in FIG. 16C, inwhich the discontinuous light extraction features 1624, such asdiffractive regions or diffusely reflective patches, are arranged in apattern that partially maps to the pattern shown in FIG. 16B. Anotherexample of a pattern of discontinuous light extraction features 1642 isschematically illustrated in FIG. 16D. It will be appreciated that manydifferent types of patterns may be used, according to the desired lightextraction profile.

One embodiment of light emitting element 2000 that uses a printedpattern of light extraction elements is schematically illustrated inFIG. 20A. A sheet 2002 is provided with an arrangement of diffuselyreflecting areas 2004 on at least one surface. Where the sheet 2002 isbrought sufficiently close to the guiding layer 610, the light 2006 inthe guiding layer 610 is coupled into the sheet 2002 and interacts withthe diffusely reflecting areas 2004. Thus, the diffusely reflectingareas 2004 may be used as light extraction features. In anotherembodiment, the diffusely reflecting areas may be provided directly onthe upper surface 611 of the guiding layer 610.

The diffusely reflecting areas 2004 may comprise, for example, a whitepigment that is printed as an arrangement of dots on the sheet 2002. Thediffusely reflecting areas 2004 may be patterned, for example so as toreduce the amount of light 2008 transmitted directly from the LED 620and to increase the lateral spreading of the light 2006 for extractionfrom the guiding layer at an increased distance from the LED 620. Thepatterning may be made, for example, by varying the thickness of thediffusely reflecting material, by varying the surface density ofdiffusely reflecting areas provided on the surface 611, by varying thedensity of the material that diffusely reflects, or some combination ofthese different approaches. In the illustrated example, the extent(surface density) of the diffusely reflecting area 2004 a above the LED620 is greater than the extent of the diffusely reflecting area 2004 bclose to the edge of the light emitting element 2000, and so the amountof diffuse reflection at the upper side of the guiding layer is greaterwhere direct illumination by the LED 620 is brighter than for otherareas of the upper side of the guiding layer where direct illuminationof the LED is less bright. The diffusely reflecting area may be somewhattranslucent, in that some light may pass through, rather than beingdiffusely reflected. Light 2008 from the LED 620 is illustrated aspassing through the diffusely reflecting area 2004 a.

Various light management film layers 2010 may be used to affect thelight once it has been directed out of the guiding layer 610. Forexample, the layers 2010 may include a layer of a brightness enhancementfilm, crossed brightness enhancement film layers, a reflective polarizerfilm, or a combination thereof. The layers 2010 may also include otherfilters and screen layers.

FIG. 20B shows another example of a light emitting element 2050, inwhich the diffusely reflecting areas 2004 are applied directly to theupper surface of the guiding layer 610. Also, the light emitting elementincludes a gap 2052 between the lower surface 612 of the guiding layer610 and the intermediate layer 622. In this case, the intermediate layer622 is reflective, so that light 2006 that is diffusely reflectedthrough the lower surface 612 is reflected back up through the guidinglayer. The reflected light 2006 may pass out of the guiding layer andthrough or between the diffusely reflecting areas 2004.

The intermediate layer 622 may be flat and parallel, for example asshown in FIG. 20A, or may be curved, for example as shown in FIG. 20B. Acurved, reflective intermediate layer may be formed by placing ESR filmover a molded form. It will be appreciate that a gap and a curvedintermediate layer may be present in the different embodiments of lightemitting element illustrated above and below.

The sheet 2002 may also be used as a diffuser to help reduce the abilityof a viewer to see the location of the LED 620 in the light. One exampleof such use is schematically illustrated in FIG. 20C. A gap 2012separates the sheet 2002 from the guiding layer 620, and light isdirected out of the guiding layer via the use of light extractionfeatures 624. The light extraction features may be any type of lightextraction features discussed above. At least some of the light 2014incident on the diffusely reflecting areas 2004 is diffusely reflectedthrough the guiding layer 610 to the intermediate layer 622. The light2014 is reflected by the intermediate layer 622, through the guidinglayer 610 and the sheet 2002. A diffuser 2016, such as a bulk diffuser,may also be used to further diffuse the light 2014, for example as isschematically illustrated in FIG. 20D.

The position and density of the diffusely reflecting areas 2004 may beadjusted so as to achieve a desired output illumination profile,regardless of whether the sheet 2002 is optically contacted to theguiding layer 610 or is separated from the guiding layer.

Light from one light emitting element may be permitted to pass to anadjacent light emitting element. However, in some applications such asinformation display, it may be desirable to prevent light from passingbetween adjacent light emitting elements. One approach to reduce suchcross-talk between adjacent light emitting elements is to ensure thatall of the light from an LED is coupled out of the guiding film beforethe light reaches the edge of the light emitting element.

Other approaches to reducing cross-talk between adjacent light emittingelements are now described with reference to FIGS. 17A-17D. In thesefigures, adjacent light emitting elements are separated by a dashed line1702. Each light emitting element comprises at least one LED 1720disposed on a substrate 1708, with an intermediate layer 1722 and aguiding layer 1710 over the intermediate layer. Light from the LEDs 1720passes into the guiding layer 1710. Diffractive light extractionelements 1724 are illustrated in each case, but other types of lightextraction elements may be used.

In the approach schematically illustrated in FIG. 17A, a reflectivebarrier 1730 is disposed between adjacent light emitting elements. Thereflective barrier 1730 may be formed using reflective material disposedin a groove in the guiding layer 1710. The reflective material maycomprise particles of a high refractive index material, such as titaniumdioxide (TiO₂), barium sulphate (BaSO₄) or aluminum oxide (Al₂O₃), in apolymer matrix such as polyester (for example PEN or PET), polymethylmethacrylate, polycarbonate, polyurethane, cyclic polyolefin, or thelike. The groove may be formed in the guiding layer when the guidinglayer is manufactured, for example by compression molding, casting andcuring, injection molding, or the like.

In the approach schematically illustrated in FIG. 17B, the reflectiveintermediate layer 1722 is provided with ribs 1740 that extend upwards.The guiding layer 1710 is formed over the intermediate layer 1722, forexample by one of the molding methods listed in the previous paragraph.The ribs 1740 act as reflective barriers between the adjacent lightemitting elements.

The LEDs 1720 need not be positioned at the center of the light emittingelement, as is now discussed with reference to FIG. 17C. For example,the LED 1720 in the left light emitting element 1750 is positioned tothe side of the light emitting element 1750, and may be placed close tothe edge or corner of the light emitting element. The light extractionfeatures 1724 are positioned and arranged to direct the light receivedin the light pattern that results from the off-center placement of theLED 1720.

In addition, LEDs 1720 may be placed at more than one location withinthe light emitting element, as is illustrated for the light emittingelement 1752 on the right hand side of the figure. In the illustratedembodiment, there are two LEDs, one on either side of the light emittingelement 1752. The light emitting element 1752 may include other numbersof LEDs 1720. For example, where the light emitting element 1752 hasfour sides, the LEDs 1720 may be positioned along the four edges of thelight emitting element 1752 or at the four corners of the light emittingelement 1752. The light extraction features 1724 are positioned andarranged in the light emitting element 1752 to direct the light receivedin the light pattern that results from the particular placement of theLEDs 1720.

In another approach, schematically illustrated in FIG. 17D, thethickness of the guiding layer 1710 is reduced at the position 1760between the two light emitting elements 1762 and 1764. Such a reductionin guiding layer thickness results in a reduction, if not elimination,of light that passes from one light emitting element to an adjacentlight emitting element within the guiding layer 1710. The reduction inguiding layer thickness may be produced, for example, by embossing theguiding layer with a groove pattern, where the grooves lie betweenadjacent light emitting elements.

Another embodiment of light emitting element 1800 is schematicallyillustrated in FIG. 18A, which shows two light emitting elements side byside. In this embodiment, LEDs 1820 are disposed on a substrate 1808. Anoptical sheet 1822 has apertures to accommodate the LEDs 1820, so thatLEDs 1820 penetrate through the apertures. Thus the substrate 1808 is onone side of the sheet 1822 and the light emitting portions of the LED1820 at least have a clear path to emit light through the apertures, ormay be through the apertures themselves, as illustrated.

The sheet 1822 is provided with a reflecting surface 1824 that reflectsthe light emitted by the diodes 1820. The reflecting surface 1824 iscurved so as to direct the light in a desired direction. For example,the reflecting surface may be paraboloidal, elliptical or have someother shape. The reflecting surface 1824 may be a metalized surfaceformed on a shaped film, or may be a multilayer reflector, for example avacuum coated dielectric reflector or a multilayer polymer reflector.The reflecting surface 1824 may be deposited on the sheet 1822. Inanother approach, the sheet 1822 itself may be formed of reflectingmaterial, for example stamped out of ESR™ film available from 3MCompany, St. Paul, Minn.

The LEDs 1820 may be flip-chip type LEDs, having both electricalcontacts on the lower surface attached to the substrate 1808, in whichcase the substrate 1808 may carry conductors for both the positive andnegative contacts of the LEDs 1820.

The space 1826 above the LED 1820 and reflecting surface 1824, may be inair or may be filled with transparent material. For example, transparentmaterial may be molded in place over the LED 1820 and reflecting surface1824. Further, a diffuser or screen film 1830 may be disposed above thesheet 1822 to diffuse the light after being reflected by the reflectingsurfaces 1824.

Another embodiment of light emitting element 1850 is schematicallyillustrated in FIG. 18B. In this embodiment, lenses are included betweenthe reflective surfaces 1824 and the diffuser or screen layer 1830. Inthe illustrated embodiment, the lenses are formed on a lens sheet 1852as prismatic lenses, for example Fresnel lenses. It will be appreciated,however, that other types of lenses may be used. For example, where thespace 1826 is filled with transparent material, the lenses may be formedon the surface of the transparent material.

The different types of light emitting elements discussed herein may eachbe incorporated in a lighting unit used, for example, for informationdisplay or for space lighting. The constructions can be very compactsince the LED dies are typically only around 300 μm thick and theguiding layer sits on top of the LED dies. Accordingly, the thickness ofsuch a construction may be only about a millimeter or two. This leads tothe possibility that the lighting unit may be flexible and may be formedin a non-planar shape. For example, the lighting unit may be wrappedaround a form, such as a cylindrical form. The lighting unit may also berigid.

While some of the embodiments of lighting unit were described above toinclude contrast/projection filters, it will be appreciated that all thedifferent types of lighting unit may be provided withcontrast/projection filters, if desired.

For a given lighting application, the brightness requirements, lamppixel count and total lamp area are all considerations when determiningthe number of LEDs required. Examples of two different lightingapplications are discussed below.

EXAMPLE 1 Ceiling Lighting

This example considers a lighting fixture having a diagonal of 70″ (178cm) and an aspect ratio of 5:1. The example considers the design of afixture that provides light equivalent to 2 fluorescent tubes at 70 L/Wand using 100 W input power, or 7000 Lumens.

The assumed LED characteristics are as shown in Table I. Thecharacteristics are similar to those for a 1 mm square Luxeon white LEDavailable from Lumileds Lighting LLC, San Jose, Calif.

The LEDs are assumed to be driven at ⅔ maximum rated power, hence theactual current is less than the maximum current. The calculated lightingunit design is summarized in Table 1. TABLE I LED CharacteristicsWavelength band White Luminous efficacy (L/W) 25 Max. forward current(mA) 350 Max. power (W) 1.19 Power consumed (W) 0.80 Actual forwardcurrent (mA) 235 Forward voltage (V) 3.4 Light out (L) 20.0

TABLE II Lighting Unit Characteristics Lightguide extraction/absorptionfactor (%) 80 LED coupling to LG (%) 95 Net efficiency (%) 76 Verticaldivergence (half max., full angle) (°) 60° Horizontal divergence (halfmax., full angle) (°) 60° Diagonal (cm) 178 Aspect ratio 5:1 Length (cm)174 Width (cm) 35 Number of LEDs 350 LED density (cm² per LED) 17.4Luminous efficiency (L/W) 25.00 Power total (W) 280 Total flux (Lumens)7000 Divergence angle gain 1.07 Axial light (candela m⁻²) 3714

Thus, in this example, a sheet of light emitting elements, with eachlight emitting element having an area of no more than 17.4 cm², with atotal area of 0.6 m², provides as much light as a pair of fluorescenttubes and associated luminaire optic that take up the same area. Thescreen is assumed to direct the light into a 60° cone, through the useof appropriate light management films and/or projection filters. Thesolid angle of the emitted light, together with the indicatedabsorption, results in an axial gain of 1.07 relative to a perfectLambertian emitter.

EXAMPLE 2 RGB Backlight for a Liquid Crystal Display

In this example, the lighting unit is used as the backlight for a liquidcrystal display (LCD) having a diagonal of 23″. Each light emittingelement includes four LED dies, one red, two green and one blue. Activecontrol of the different LED dies permits for control of the backlightcolor. In the example design, the backlight has a color temperature of6500 K. TABLE III LED Characteristics for Backlight Total Number/colortriad 1 2 1 4 Wavelength band Red Green Blue Luminous efficacy (L/W) 4023 5 18.81 Max. forward current (mA) 385 700 350 Max. power (W) 1.142.39 1.2 4.73 Actual forward current (mA) 126 505 320 Power consumed (W)0.37 1.73 1.09 3.19 Forward voltage (V) 2.95 3.42 3.42 Light out (L)14.83 39.70 5.47 60 % of luminous content 24.7 66.2 9.1 Colortemperature (K) 6500

The lighting unit is assumed to use a single sheet of brightnessenhancing film and to produce a horizontal viewing angle of 56° (halfmaximum, half angle) and a vertical viewing angle of 37° (half maximum,half angle). The total flux emitted from the backlight is about 1560lumens, and the on-axis brightness is about 5191 candelas/m². Thecalculated properties of the example backlight unit are listed in TableIV. TABLE IV RGB Backlight Characteristics Lightguideextraction/absorption factor (%) 80 LED coupling to LG (%) 95 Netefficiency (%) 76 Vertical divergence (half angle)(°) 37° Horizontaldivergence (half angle) (°) 56° Diagonal (cm) 58 Aspect ratio 1:1.78Length (cm) 51 Width (cm) 28.6 Number of RGB LED triad cells 26 No. ofLED dies 104 LED density (cm² per LED triad cell) 56.1 Luminousefficiency (L/W) 18.81 Power total (W) 82.95 Total flux (Lumens) 1560Absorption adjusted viewing area gain 1.60 Backlight axial light(candela m⁻²) 5191

Thus, the present invention may effectively be used as a backlight forLCD displays. Such a backlight may permit the LCD display to operatewith field sequential color, that is, the sequential illumination of theLCD with light of different colors. Such an approach to illuminating theLCD eliminates the need for color filters in color LCD displays, thusincreasing the overall efficiency and reducing cost. Opticallycompensated bend (OCB) mode LCDs are particularly useful for operatingin a field sequential color illumination mode, due to their fastresponse times.

In addition, a backlight as described may be controlled so thatdifferent areas of the display are illuminated at different levels ofintensity. This may be advantageous, for example, when displaying animage having high contrast, where one part of the image is very brightand another part of the image is very dark. The brightness of the LEDsilluminating the dark part of the image may be reduced, or they may evenbe turned off, with the result that the dark areas of the image appeareven darker.

The present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art to which the presentinvention is directed upon review of the present specification. Theclaims are intended to cover such modifications and devices.

1. An optical assembly for emitting light, comprising: an array ofinorganic light emitting diodes (LEDs) mounted to a substrate, the LEDsemitting light in a direction generally perpendicular to the substrate;and an optical sheet disposed over the LEDs, at least a portion of lightentering one side of the optical sheet from the LEDs being guided withinthe optical sheet in a direction generally parallel to the substrate. 2.An assembly as recited in claim 1, wherein the optical sheet comprises afirst side facing towards the substrate and a second side facing awayfrom the substrate, at least one light extraction feature being providedwith the optical sheet to extract light, that has been guided along theoptical sheet from the LEDs, through one of the first and second sides.3. An assembly as recited in claim 2, wherein the light extractionfeature comprises one or more light extraction prisms having prism basesdisposed on the second side of the optical sheet.
 4. An assembly asrecited in claim 3, wherein the light extraction prisms are arranged onthe second side of the optical sheet as one or more Fresnel lenses. 5.An assembly as recited in claim 3, wherein the light extraction prismson the second side are substantially linear.
 6. An assembly as recitedin claim 3, wherein the light extraction prisms on the second side arecurved.
 7. An assembly as recited in claim 2, wherein the lightextraction feature comprises one or more diffractive structures on atleast one of the first and second sides of the optical sheet.
 8. Anassembly as recited in claim 2, wherein the light extraction featurecomprises one or more patches of diffusely reflecting material on atleast one of the first and second sides of the optical sheet.
 9. Anassembly as recited in claim 2, wherein the light extraction featurecomprises one or more indentations on one of the sides of the opticalsheet.
 10. An assembly as recited in claim 2, wherein the lightextraction feature comprises one or more grooves on the second side,light guided within the optical sheet being directed by the one or moregrooves to the first side of the optical sheet.
 11. An assembly asrecited in claim 2, further comprising a reflecting layer disposedbetween the optical sheet and the substrate so that light, directed tothe reflecting layer by the extraction feature, is reflected by thereflecting layer out of the optical sheet.
 12. An assembly as recited inclaim 2, wherein the extraction feature comprises an array of lightextraction prisms disposed with respective prism apexes directed towardsthe optical sheet.
 13. An assembly as recited in claim 12, furthercomprising a coupling layer on the second side of the optical sheet,wherein the guided light passes into the coupling layer and the prismapexes extend into the coupling layer.
 14. An assembly as recited inclaim 12, wherein light that has coupled from the optical sheet into thelight extraction prisms is totally internally reflected by the prisms ina direction away from the optical sheet.
 15. An assembly as recited inclaim 12, further comprising an optical diffuser, light that has passedthrough the light extraction prisms from the optical sheet being passedthrough the optical diffuser.
 16. An assembly as recited in claim 12,further comprising a filter, light that has passed through the lightextraction prisms from the optical sheet being passed through thefilter.
 17. An assembly as recited in claim 16, wherein the filtercomprises a projection screen.
 18. An assembly as recited in claim 16,wherein the filter comprises a polarizer.
 19. An assembly as recited inclaim 12, wherein the light extraction prisms are substantially linear.20. An assembly as recited in claim 12, wherein the light extractionprisms are curved.
 21. An assembly as recited in claim 20, wherein thecurved light extraction prisms are curved in patterns centered onrespective LEDs.
 22. An assembly as recited in claim 2, wherein theextraction feature comprises a portion of at least one of the sides ofthe optical sheet that lies nonparallel to the substrate.
 23. Anassembly as recited in claim 2, wherein the extraction feature comprisesa portion of the optical sheet where at least one of the first side andthe second side of the optical sheet has a surface non-parallel to thesubstrate.
 24. An assembly as recited in claim 23, wherein the lightinternally reflects at the surface non-parallel to the substrate.
 25. Anassembly as recited in claim 23, wherein the surface non-parallel to thesubstrate is provided with a surface structure that directs lightincident thereon towards an opposing side of the optical sheet.
 26. Anassembly as recited in claim 25, wherein the surface structure comprisesone or more turning prisms.
 27. An assembly as recited in claim 25,wherein the surface structure comprises one or more surface steps. 28.An assembly as recited in claim 23, wherein the second side of theoptical sheet contains one or more indented surfaces disposed aboverespective LEDs, so that light from the LEDs that passes through theoptical sheet from the LEDs in a direction substantially normal to theoptical sheet is internally reflected at the one or more indentedsurfaces.
 29. An assembly as recited in claim 2, wherein the extractionfeature comprises one or more surface structures disposed on the firstside of the optical sheet.
 30. An assembly as recited in claim 29,wherein the one or more surface structures on the first side of theoptical sheet comprise angled reflective faces so that light, that hasbeen guided within the optical sheet, is directed by the angledreflective faces to be incident on the second side of the optical sheetat an angle that permits transmission of the light through the secondside.
 31. An assembly as recited in claim 29, wherein the one or moresurface structures on the first side of the optical sheet comprisediffusive elements that diffusely reflect light incident thereon.
 32. Anassembly as recited in claim 2, wherein the extraction feature comprisesat least one continuous light extraction feature surrounding arespective LED.
 33. An assembly as recited in claim 2, wherein theextraction feature comprises at least one light extraction feature thatdoes not continuously surround its respective LED.
 34. An assembly asrecited in claim 2, wherein the extraction features comprise areas ofdiffusely reflecting material.
 35. An assembly as recited in claim 34,wherein the diffusely reflecting material is arranged so as to provide agreater degree of diffuse reflection at positions illuminated byrelatively more intense light from the LEDs than for positionsilluminated by relatively less intense light from the LEDs.
 36. Anassembly as recited in claim 1, further comprising light managementoptics to direct light extracted from the optical sheet.
 37. An assemblyas recited in claim 36, wherein the light management optics comprise atleast a first brightness enhancing prismatic film having an array ofprisms arranged to direct light extracted from the optical sheet in adirection more parallel to an axis normal to the optical sheet.
 38. Anassembly as recited in claim 36, wherein the light management opticsfurther comprises a second brightness enhancing optical film having anarray of prisms arranged to direct light extracted from the opticalsheet in a direction more parallel to the axis normal to the opticalsheet, the prisms of the second brightness enhancing optical filmextending in a direction perpendicular to an extension direction of theprisms of the first brightness enhancing sheet.
 39. An assembly asrecited in claim 1, further comprising a projection screen disposed overthe optical sheet so that light extracted from the optical sheet passesthrough the projection screen.
 40. An assembly as recited in claim 1,wherein the optical sheet comprises contains one or more elements todeflect some of the light from the LEDs, that would otherwise passthrough the optical sheet, within the optical sheet.
 41. An assembly asrecited in claim 40, wherein the one or more elements comprise indentedsurfaces disposed above respective LEDs, so that some of the light fromthe LEDs that passes through the optical sheet from the LEDs in adirection substantially normal to the optical sheet is internallyreflected within the optical sheet at the one or more indented surfaces.42. An assembly as recited in claim 40, wherein the one or more elementscomprise one or more diffusers disposed with the optical sheet todiffusely direct some of the light from the LEDs within the opticalsheet.
 43. An assembly as recited in claim 1, wherein the optical sheetgenerally lies parallel to the substrate.
 44. An assembly as recited inclaim 1, wherein the LEDs are disposed on the substrate as LED dies. 45.An assembly as recited in claim 1, further comprising an intermediatelayer disposed between the optical sheet and the substrate.
 46. Anassembly as recited in claim 45, wherein the intermediate layer isreflective.
 47. An assembly as recited in claim 45, wherein theintermediate layer is provided with at least one light extractionfeature to extract light that has been guided along the optical sheetfrom the LEDs.
 48. An assembly as recited in claim 47, wherein the lightextraction feature comprises one or more angled surfaces non-parallel tothe substrate.
 49. An assembly as recited in claim 47, wherein the lightextraction feature comprises one or more patches of diffusely reflectivematerial.
 50. An assembly as recited in claim 45, further comprising anarrangement of areas of diffusely reflecting material disposed todiffusely reflect at least some of the light that has passed out of theoptical sheet.
 51. An assembly as recited in claim 50, wherein theintermediate layer is reflective, and some of the light diffuselyreflected by the areas of diffusely reflecting material is reflected bythe reflective intermediate layer so as to pass between the areas ofdiffusely reflecting material.
 52. An assembly as recited in claim 51,further comprising a diffuser disposed to diffuse light that has passedbetween the areas of diffusely reflecting material.
 53. An assembly asrecited in claim 1, wherein the substrate is reflective.
 54. An assemblyas recited in claim 53, wherein the substrate is provided with at leastone light extraction feature to extract light that has been guided alongthe optical sheet from the LEDs.
 55. An assembly as recited in claim 53,wherein the light extraction feature comprises one or more angledsurfaces non-parallel to the substrate.
 56. An assembly as recited inclaim 53, wherein the light extraction feature comprises one or morepatches of diffusely reflective material.
 57. An assembly as recited inclaim 1, wherein the optical sheet comprises electrical conductorsdisposed on a side of the optical sheet facing the LEDs, the electricalconductors connecting to emitting sides of respective LEDs.
 58. Anassembly as recited in claim 1, wherein the substrate compriseselectrical conductors connecting to base sides of respective LEDs. 59.An assembly as recited in claim 1, further comprising an intermediatelayer between the optical sheet and the substrate, and electricalconductors on the intermediate layer connecting to respective LEDs. 60.An assembly as recited in claim 1, wherein the assembly is flexible soas to be formable into a non-planar shape.
 61. A light emitting systemhaving a plurality of individually illuminated light emitting elements,the system comprising: an array of inorganic light emitting diodes(LEDs), different LEDs corresponding to respective light emittingelements of the light emitting system; a light spreader sheet disposedover the LEDs, light entering the light spreader sheet from the LEDsbeing spread transversely within the spreader sheet over an areacorresponding to the respective light emitting elements of the lightemitting system, the light spreader sheet comprising light directingfeatures that direct the spread light out of the spreader sheet.
 62. Asystem as recited in claim 61, further comprising a power supplyconnected to supply power to the LEDs.
 63. A system as recited in claim61, further comprising a control unit coupled to control opticalemission of the LEDs.
 64. A system as recited in claim 63, wherein thecontrol unit controls individual LEDs to emit independently of otherLEDs, so that the light emitting system has an illuminated area thatconveys information to a viewer.
 65. A system as recited in claim 61,wherein each light emitting element is associated with at least one LED.66. A system as recited in claim 65, further comprising a colorconverter disposed with the at least one LED to produce color convertedlight from light emitted by the at least one LED.
 67. A system asrecited in claim 65, wherein each light emitting element is associatedwith three LEDs mounted together, the three LEDs being adapted to emitlight in three different wavelength ranges.
 68. A system as recited inclaim 61, further comprising a projection element disposed to projectlight, received from the spreader sheet, dispersed over a desiredviewing angle.
 69. A system as recited in claim 68, wherein theprojection element comprises a diffusive screen.
 70. A system as recitedin claim 68, wherein the projection element comprises a refractivescreen.
 71. A system as recited in claim 61, further comprising at leastone light blocking element disposed between adjacent light emittingelements to reduce light generated in one of the light emitting elementsfrom passing to an adjacent light emitting element.
 72. A system asrecited in claim 71, wherein the light blocking element comprises agroove formed in the light spreader sheet.
 73. A system as recited inclaim 72, wherein the groove is filled with a reflective material.
 74. Asystem as recited in claim 72, wherein the groove is formed in a surfaceof the light spreader sheet facing towards the LEDs.
 75. A system asrecited in claim 72, wherein the groove is formed in a surface of thelight spreader sheet facing away from the LEDs.
 76. A light emittingsystem having a plurality of individually illuminated light emittingelements, the system comprising: an array of inorganic light emittingdiodes (LEDs) emitting light generally in a light emission direction;light spreading means for laterally spreading light in a directionacross the array of the LEDs; and light directing means for directinglight from the light spreading means in a desired illuminationdirection.
 77. An assembly for emitting light, comprising: an array ofinorganic light emitting diodes (LEDs) arranged on a substrate to emitlight generally in a light emission direction; an array of reflectorsdisposed with the LEDs, the reflectors defining individual portions of areflector sheet, the reflectors having respective apertures, respectiveLEDs of the array of LEDs protruding through the respective apertures,the substrate being positioned to a first side of the reflector sheet,light emitting surfaces of the LEDs being positioned to a second,reflecting side of the reflector sheet; and a screen layer disposed onthe second side of the reflector sheet, at least some of the light fromthe LEDs being directed by the screen layer after reflecting off thecurved reflectors.
 78. An assembly as recited in claim 77, wherein thereflector sheet comprises a reflector sheet base having a surface withcurved regions, and a reflective layer disposed on the curved regions ofthe reflector sheet base.
 79. An assembly as recited in claim 78,wherein the reflective layer comprises multiple polymer layers ofalternating refractive index.
 80. An assembly as recited in claim 77,wherein the reflectors are curved with a generally paraboloidal curve.81. An assembly as recited in claim 77, wherein the substrate comprisesconductors for carrying electrical current between the LEDs and a powersource.
 82. An assembly as recited in claim 77, wherein at least some ofthe LEDs are provided on the substrate as LED dies.
 83. An assembly asrecited in claim 82, wherein at least one of the LED dies is a flip-chipLED die, and the substrate comprises a first conductor coupled to afirst electrode of the flip-chip LED die and a second conductor coupledto a second electrode of the flip-chip LED die.
 84. An assembly asrecited in claim 77, wherein the screen layer comprises a diffusersheet.
 85. An assembly as recited in claim 77, wherein the screen layercomprises a screen having refracting elements.
 86. An assembly asrecited in claim 85, wherein the screen having refracting elementscomprises a screen with a lensed surface.
 87. An assembly as recited inclaim 85, wherein the screen having refracting elements comprises abeaded screen.
 88. An assembly as recited in claim 77, furthercomprising a light directing sheet disposed between the reflector sheetand the screen layer.
 89. An assembly as recited in claim 88, whereinthe light directing sheet comprises refracting surface features torefract light passing therethrough.